A person whose mouth might as well be an
anus because all that comes out of it is
bullshit.
The
joke is that since shit comes out of an ass, and name-calling and spreading slander or lies are called talking shit, that means their mouth is an anus since shit-quality words are all that comes out of it.
Bob: Climate change is a hoax.
Alice: What? No it isn't! Haven't you noticed the jet stream weakening?
Bob: Also, vaccines don't
work.
Alice:
Wow, you're an assmouth. Try saying something LOGICAL next
time.
Bob: Hmmphh! Fine!